1. Field of the Invention
The present invention relates to a mobile station of a mobile-communication system, for example, mobile-communication equipment such as a cellular telephone and a personal digital assistance (PDA), and to a transmission-power control method thereof.
2. Description of the Related Art
One multiple-access method adopted in the mobile-communication system is CDMA (Code Division Multiple Access) method. The CDMA method transmits information between a plurality of mobile stations and base stations, using the same frequency. This case may cause a so-called near-far problem in that, if each mobile station transmits a radio wave at the same transmission power, the radio wave from a mobile station that is further from the base station has less power that is received at the base station than a radio wave from a mobile station that is nearer the base station, so that the radio wave from the mobile station further from the base station is strongly affected by other radio waves, thereby degrading communication quality. To resolve this near-far problem, each mobile station accurately controls the transmission power to ensure that the constant power arrives at the base station. This transmission-power control can reduce the interference between the communication channels of each mobile-station to increase the frequency-utilization efficiency.
The recent wideband CDMA (W-CDMA) method defines a maximum permissible value and accuracy of the transmission power. More specifically, the transmission-power control in the W-CDMA method requires a wide range of transmission-power control that uses “1 dB step” control (which means one control-step-width being 1 dB). A transmission-power control technique is thus needed that can provide a higher maximum-permissible value and a higher accuracy of the transmission power.
The transmission-power control may generally have a reduced absolute-accuracy when power is transmitted at higher output conditions due to the poor linearity of input/output characteristics (the characteristics of input power vs. output power) of the power amplifier (power amp). Various methods have thus been proposed that can provide a wider range of transmission-power control and a higher accuracy power control. One approach is a transmission-power control method with a feedback system, such as PAC (Automatic Power Control) and ALC (Automatic Level Control) (see Japanese application patent laid-open publications No. Hei 11-308126 and Hei 07-307631). This transmission-power control with the feedback system provides feedback to allow the power amplifier, which amplifies the transmission signal, to output a specified transmission-power set value. For example, the feedback system with digital processes detects and measures the transmission power, and compares the detection results and the transmission-power set value to determine the transmission-power error. The feedback system then multiplies the determined transmission-power error by a predetermined loop-gain value to determine an error value, and integrates the determined error value to calculate a feedback amount. The feedback amount thus calculated is reflected as the transmission-power adjustment.
The following problems arise, however, in the above-mentioned conventional transmission-power control techniques.
In the feedback system with digital processes, a larger loop-gain value can increase each feedback amount, so that the transmission-power value can reach the set value in a shorter time, but the transmission-power value does not converged due to the control delay and the like and consequently, the feedback system may oscillate. On the other hand, a smaller loop-gain value can decrease each feedback amount to prevent the oscillation problem, but the transmission-power value may reach the set value in a longer time. Additional problems may arise in which the transmission-power error may not be appropriately corrected with the feedback amount because of the combined effects of the operation error of the feedback value associated with the digital processes (which is a truncation error due to the rounding and referred to hereafter for simplicity as a rounding error) and the temperature characteristics of PA and detector and the like.
A more detailed description is given below of the problems with the combined effects of the rounding error and the temperature characteristics.
Diode detection is generally used to detect the transmission power. The diode detection measures the transmission-power by detecting the amplitude of the transmission-power waveform, so that the detected voltage values which are results of the measurement are true values (voltages). The transmission-power set value is usually given in “dB”, which is a unit of power, so that the feedback system includes circuitry based on “dB”. In such a circuit, a conversion table is used for converting the detected voltage value to the power value (dB). The conversion table is usually created based on the correlation between the actually-detected voltage value and the transmission-power value. Such a correlation is acquired based on the results that are obtained by measuring the diode characteristics under the condition in which there is no temperature-change effect, for example, the condition in which the temperature of the radio transmitter is stable.
The feedback system using the conversion table as mentioned above is affected by temperature characteristics, particularly of the PA affect. Upon start of transmission, a large amount of current starts to flow through PA, which then heats up and its temperature increases. The temperature increase of PA is the largest shortly after the start of transmission, and then decreases with time to reach a constant PA temperature. The conversion table is created based on the condition in which the PA temperature is constant.
PA has semiconductor characteristics that provide higher outputs at lower operation temperatures, and lower outputs at higher operation temperatures. After the transmission starts, therefore, the transmission-power value which is outputted by PA gradually decreases as the PA operation-temperature is increased. For PA to provide a constant output, it is necessary to increase an input of PA as the operation temperature increases. Conventional transmission-power control techniques, however, do not increase the PA input with the temperature increases. During the period from the start of the transmission until the PA temperature is stable, therefore, the power value that is converted based on the above-mentioned conversion table includes an error due to the PA temperature characteristics, and the combination of this error and the rounding error may mistakenly make the error value (feedback amount) zero. Once the error value becomes zero, the feedback system subsequently does not accumulate the feedback amount or does not reflect the error, thereby providing an incorrect feedback control which may send the transmission signal at the incorrect transmission power.
Japanese application patent laid-open publication No. Hei 11-308126 has proposed a feedback system which holds a plurality of loop gains and switches between a high-speed mode for a shorter convergence time and a high-accuracy mode for higher accuracy control. In this case, the static modes are switched in the feedback system, so that PA temperature characteristics and the like are not taken into account and a problem may arise due to the combined effects of the rounding error and the temperature characteristics as mentioned above.
As a conversion table (true value (voltage) to power), a plurality of tables may be provided based on the characteristics of each temperature range. More specifically, conversion tables may be created at four stages of −25° C. to 0° C., 0° C. to 25° C., 25° C. to 50° C., and 50° C. or more, and the tables may be switched for each measurement temperature. This case, however, may require a larger-scale circuit for the increased number of tables. An additional problem may arise in which the discontinuity of the transmission-power value, when conversion tables are switched, may make the control more difficult.